A liquid epoxy resin composition is widely used in various fields such as, for example, adhesives, casting materials, sealants, laminating material, molding materials and coating materials, because of its excellent cured property and easy handling.
Recently, resin materials have been required to have high-performance characteristics with the advance of related techniques and the variation of the environments where they are used. In particular, where resin materials are used as adhesives, casting materials, sealants, molding materials, laminating materials, improvement in the heat resistance and water resistance of the materials is important. However, conventional liquid epoxy resin compositions which have been commonly used up to the present do not sufficiently satisfy the necessary characteristics.
For instance, liquid epoxy resin compositions containing, as a base, a bisphenol A-type epoxy resin or bisphenol F-type epoxy resin, which are widely used at present, do not have sufficient heat resistance and water resistance.
In order to improve the heat resistance, multi-functional epoxy resins such as novolak-type epoxy resins, tri-functional epoxy resins, and tetra-functional epoxy resins were used. However, they are difficult to handle as liquids since such multi-functional epoxy resins have extremely high viscosities or are solids at room temperature.
Liquid epoxy resins having excellent heat resistance and having a low viscosity derived from amine compounds such as aminophenols, xylenediamines are known. However, cured resins of such amine-type epoxy resins have poor water resistance.
Epoxy resins with a rigid skeleton such as, for example, biphenol-type epoxy resins, bisphenol S-type epoxy resins, hydroquinone-type epoxy resins and the like, have excellent heat resistance and a low molecular weight are expected to have a low viscosity. However, because of their rigid skeletons, they often crystallize at room temperature. Therefore, it is difficult to handle them as liquids.
Japanese Patent Application Laid-Open No. 4-211420 discloses a method of reacting a liquid epoxy resin and an unsaturated carboxylic acid as a mean of preventing the crystallization of liquid epoxy resins. However, the method is not effective enough to liquify crystalline epoxy resins. In this method, when the degree of modification of the epoxy resin is raised (or the amount of the unsaturated carboxylic acid to be used for the modification is increased) to increase the crystallization preventing effect, then the number of the functional groups in the epoxy resin is decreased resulting in worse heat resistance and water resistance properites of the cured resin.
Japanese Patent Application Laid-Open No. 3-14816 discloses a method of inactivating the crystallizing property of biphenol-type epoxy resins, in which a biphenol-type epoxy resin is reacted with a polyphenol. However, since the object of the method was to increase the molecular weight of a biphenol-type epoxy resin to modify the resin into an amorphous solid with a low softening point and a high viscosity for the purpose of facilitating the transfer molding of the modified resin, this method does not porvide a liquid resin composition with a low viscosity.
It is therefore an object of the present invention to provide a novel modified liquid epoxy resin composition which has a low viscosity and is a stable liquid at low temperature. It is another object of the invention to provide a cured resin having excellent heat resistance and water resistance. It is another object of the present invention to provide a novel curable liquid epoxy resin composition comprising the modified liquid epoxy resin composition and an epoxy resin curing agent.